Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials.
The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer). Free carrier pairs are created by light energy and then separated by the p-n heterojunction to produce an electrical current.
In order to collect the electrical current generated by the p-n heterojunction, a conductive layer is typically present between the window glass and the CdS layer, and a back contact layer is typically attached to the CdTe layer opposite of the CdS layer. The conductive layer and the back contact serve as negative and positive contacts, respectively, in the device. Typically, the conductive layer in CdTe-based devices includes cadmium stannate (Cd2SnO4) to form a transparent conducting oxide layer on the glass. Low sheet resistance and high transmission in the solar spectrum give cadmium stannate films the desired properties for a front (i.e., negative) contact.
The transparent conductive oxide (“TCO”) layers including cadmium stannate are typically sputtered onto an unheated glass substrate resulting in an amorphous film, which is then annealed in an inert ambient at a pressure lower than atmosphere in the presence of CdS at a temperature above 600° C. for at least 20 minutes. After annealing, the film has a high transmission and low sheet resistance.
While this method is acceptable in a laboratory setting, such a high temperature anneal is difficult, if not prohibitive, in a manufacturing setting during commercial-scale production. For example, it is difficult to uniformly heat the glass superstrate to ensure that a uniform TCO layer is formed. The difficulty of uniformly heating the glass superstrate is amplified when large modules (e.g., having a surface area of greater than about 0.5 m2) are being formed, which is common in a manufacturing setting during commercial-scale production.
Thus, a need exists for methods of forming a uniform TCO layer on a glass superstrate in order to efficiently manufacture cadmium telluride photovoltaic devices in commercial-scale production.